Subcutaneous drug delivery is employed for treatment of conditions such as diabetes, and typically involves modalities such as syringe injections, pre-filled pen injectors and patient-filled portable insulin pumps. Pre-filled pen injectors provide accurate manual insulin dosing using, for example, a pre-filled, bubble-free glass cartridge. Since the glass cartridges are bubble-free, the priming process is simple for the patient. Since the injection is performed manually, however, patient compliance is a challenge; the patient may not observe proper injection timing and/or fail to follow the dosing prescription. Portable insulin pumps can provide fully controlled insulin delivery, improving patient compliance, and reduced numbers of injections (once every 3 days, for example) and programmable dosing schedules enhance the patient's quality of life. Patch pumps with low pump profiles can be attached to the patient's skin without interfering with daily activities such as including showering, sleeping, and exercising. Because these pumps are typically filled by patients, however, risks arise during the priming procedure. Improperly primed reservoirs may contain large air bubbles and cause the pump to inject too much air into the subcutaneous tissue, which poses serious safety concerns.
Accordingly, portable pumps with small footprints and pre-filled drug reservoirs can address various problems including those discussed above. One of the challenges for pumps utilizing glass vials as drug reservoirs is to provide controlled and accurate drug delivery. This challenge arises due to varying stiction/friction forces between the surface of the plunger and glass vial. Even under the same driving pressure, these variable forces may cause the drug to be delivered at different flow rates for basal (continuous) delivery. It will also make bolus delivery (i.e., delivery of a discrete dose over a short time period) unpredictable from one bolus to the next.
A related problem observed in connection with piston-driven pumps is a characteristic residual “tailing” of the flow rate—that is, if the amount of fluid expelled is plotted as a function of time, the plot will contain an asymmetric peak having a steepened front portion and an extended tail portion. This is evident, for example, in many drug-delivery devices that contain a drug reservoir formed of compliant materials. This tailing effect leads to a longer delivery time and, once again, inaccuracies in delivery volume.